<head>
    <style>
        #div_outer {
            width: 100%;
            position: absolute;
            top: 100px;
            left: 0;
        }

        #div_inner {
            background: lightgrey;
            height: 40px;
            box-shadow: 0 0 5px 5px darkblue;
        }
    </style>
</head>

<body>
    <div id="div_outer">
        <div id="div_inner">
        </div>
    </div>
</body>
